Superconductivity is usually defined as the complete loss of electrical resistance of a material at a well-defined temperature. It is known to occur in many materials, including about a quarter of the elements of the periodic table and over 1000 alloys and other multi-component systems. Generally, superconductivity is considered to be a property of the metallic state of a material since all known superconductors are metallic under the conditions that cause them to be superconducting. A few normally non-metallic materials, for example, become superconducting under very high pressure wherein the pressure converts them to metals before they exhibit superconducting behavior.
Superconductors are known to be very attractive for the generation and energy-saving transport of electrical power over long distances, and as materials used to form the coils of very strong magnets. These magnets are used in, for example, plasma and nuclear physics, nuclear magnetic resonance medical diagnosis systems, and in connection with the magnetic levitation of fast trains. Other potential uses of superconducting materials occur in power generation systems using thermonuclear fusion where very large magnetic fields must be provided, superconducting magnets being the only possible means for providing such high fields. In addition to these applications, superconductors are known in high speed switching devices, such as Josephson type switches, and in high density packaging and circuit layouts. Superconductors also are used in different types of electronic instrumentation, such as magnetic susceptometers and magnetometers.
While the advantages of superconductors are quite obvious to scientists and engineers, the common disadvantage of all presently known superconductive materials lies in their very low transition temperature. This temperature is often called the critical temperature Tc and is the temperature above which superconductivity will not exist. Usually Tc is on the order of a few degrees Kelvin. The element with the highest Tc is niobium whose Tc is 9.2° K. The composition having the highest previously known Tc is Nb3Ge which exhibits a Tc of about 23° K at ambient pressure. Transition metal alloy compounds of the A15(Nb3Sn) and B1(NbN) structure have been shown to have high superconducting transition temperatures. Among the A15 compounds is the aforementioned composition Nb3Ge. Some of these compositions are described in J. Muller, Rep. Prog. Phys. 43, 663 (1980), and M. R. Beasley et al, Phys. Today, 37 (10), 60 (1984).
It is known in the art that a small number of oxides will exhibit superconductivity. Reference is made to D. C. Johnston et al, Mat. Res. Bull. 8, 777 (1973), which describes high temperature superconductivity in the Li—Ti—O system with superconducting onsets as high as 13.7° K. These materials have multiple crystallographic phases including a spinel structure exhibiting the high Tc. Other metallic oxides, such as the perovskite Ba—Pb—Bi—O system can exhibit superconductivity due to high electron-phonon coupling in a mixed valent compound, as described by G. Binnig et al, Phys. Rev. Lett., 45, 1352 (1980), and A. W. Sleight et al, Solid State Communications, 17, 27 (1975).
As is evident from the foregoing, superconductors presently known require liquid helium for cooling and this, in turn, requires an elaborate technology and a considerable investment in cost and energy. Accordingly, it is a primary object of the present invention to provide new compositions which exhibit high Tc and methods for using and producing the same.
It is another object of the present invention to provide new superconducting compositions and methods for using and making them where cooling with liquid helium is not required in order to have superconductive properties in the compositions.
It is another object of the present invention to provide novel superconductive materials that are multi-valent oxides including transition metals, the compositions having a perovskite-like structure.
It is a further object of the present invention to provide novel superconductive compositions that are oxides including rare earth and/or rare earth-like atoms, together with copper or other transition metals that can exhibit mixed valent behavior.
It is a still further object of the present invention to provide novel superconductive compositions exhibiting high Tc, where the compositions are oxides including a phase having a layer-like structure and including copper.
It is a still further object of the present invention to provide new superconductive compositions exhibiting high Tc, where the superconductive compositions include layered structures including a rare earth and/or rare earth-like element and a transition metal.
It is another object of this invention to provide a new class of superconducting compositions characterized by a Tc greater than 26° K, and methods for making and using these compositions.
It is another object of this invention to provide new compositions and methods for using them, where the compositions include a multi-valent oxide of copper and exhibit a Tc greater than 26° K.
The basis for our invention has been described by us in the following previously published article: J. G. Bednorz and K. A. Muller, Zeitschrift fur Physik B—Condensed Matter, 64, pp. 189-193.
Another article of interest by us is J. G. Bednorz, K. A. Muller, M. Takashige, Europhysics Letters, 3(3), pp. 379-385 (1987).